This invention relates to an improved cannula for communication with the vascular system in order to gain access to a supply of blood for extracorporeal blood processing, for example hemodialysis, hemoperfusion, or any other desired form of extracorporeal blood treatment.
In the early days of dialysis therapy, extracorporeal blood flows were moderate, at about 200 ml/min. with the result that a dialysis treatment took six hours or more. These treatments, while long, were often uneventful, not least because the extracorporeal pressures were moderate with the low flow rates, typically between xe2x88x92120 mm./Hg (arterial) and +120 mm./Hg (venous). Modern therapy has increased the flow rates to as much as 500 ml./min. or more, with corresponding increases in treatment efficiency. However, the operating pressures have also increased, even to as much as xe2x88x92400 mm./Hg (arterial) and as much +600 mm./Hg (post pump). At these pressures, undesirable events can take place more readily, for example, hemolysis, foaming or clotting of the blood, air emboli, and other alarm conditions, which may be frequent and sometimes severe.
While the operating pressures could be reduced by increasing the inner diameter of the patient access cannulae, which are typically the narrowest portion of the extracorporeal blood circuit, an increase in the size of the needles, which could significantly reduce pressures, is strongly objected to by the patients. A large needle results in a larger incision. Patients have historically shown reluctance to be penetrated in a fresh site by a needle with a cutting point larger than 15 G (gauge).
By this invention, a vascular cannula is provided, which is capable of passing high blood flow rates at reduced pressures without significant enlargement of the distal end portion of the cannula that penetrates the patient""s skin and vascular system. The cannula of this invention is particularly desirable for use in conjunction with implanted artificial access ports for the vascular system. For example, as shown in Finch et al. U.S. Pat. No. 5,562,617, Enegren et al. U.S. Pat. No. 4,955,861, or International Publication WO97/47338.
By this invention, a cannula is provided for communication typically with the vascular system of the patient. The cannula has a proximal end connected with a blood flow tube, the cannula having an inward taper between the proximal end and a distal end, whereby the distal end is of less diameter than the proximal end. The distal end is pointed, beveled or not beveled, but is blunt enough to be effectively incapable of cutting through intact, human skin (for example at forces of less than 100 gm.) being preferably advanced through the skin to an implanted vascular access port by means of a preformed track through the tissue that does not require a sharp forward cutting edge on the cannula, or an accompanying trocar, in a manner similar to that disclosed in Vasca Inc. International Publication No. WO99/03527.
Preferably, the cannula is rigid, being made of a surgical steel, copolymer plastic, or the like, in which the inward taper extends substantially the entire cannula length.
Thus, substantial portions of the cannula have an enlarged inner diameter over that which penetrates the patient, which can have the effect of greatly reducing flow resistance through the system (since pressure resistance has a fourth power, inverse relationship to the inner diameter of a flow passageway).
The subsequent discussion of gauges and tapers is relevant for hemodialysis wherein flows are relatively high, (for example from 180 ml/min to 60 ml/min and more. This invention is also valuable for other low-flow applications such as chemotherapy (for example, from 20 ml/min to 0.5 ml/min or less). In such latter case a properly sized tapered cannula may preferably be from 25 G to 21 G, or anything in between, at its distal end. Other uses of implanted, artificial ports and tapered, blunt access cannulas may be for diabetes therapy, urinary ports, and the like, with different flow rates and cannula sizes contemplated, for example 15 G to 19 G at the distal end.
The taper may preferably define an angle of 1-3 or 4 degrees (per side) to the cannula longitudinal axis. Also preferably, the distal cannula end may be of 11-13 gauge and the proximal cannula end may be 14-15 gauge. The xe2x80x9cgaugexe2x80x9d measure is the well-known, commonly used system of the industry.
If desired, the cannula may have a single lumen, although multiple lumen cannulas may be used as well, and the lumen may receive a removable trocar. The trocar may have a blunt forward end, also effectively incapable of cutting through living tissue, to pass with the cannula along a preformed tissue track through the skin to an implanted vascular access port. Alternatively, the trocar may be straight or tapered, and may have a sharp end to assist the blunt-end cannula in advancing through tissue into communication with the vascular system of a patient.
As a further preferable advantage of a blunt non-beveled tapered cannula, the blunt cannulas can be made in mass production, but it has been technically and economically impractical to grind sharp or dull, beveled ends on tapered cannulas with conventional manufacturing equipment. Cannulas are typically ground in batches of two hundred or more while set in a jig. This is not practical in the conventional equipment with tapered cannulas. Accordingly, the use in accordance with this invention of a blunt, non-beveled, tapered cannula makes possible the large scale, commercial manufacture thereof with conventional manufacturing equipment, while achieving the advantages of reduced flow pressures at higher flow rates, which can be obtained by the use of tapered vascular cannulas.
Also, by this invention, external fluid connection with an implanted artificial port communicating with a body lumen of a patient may be achieved. The artificial port used in this invention may have an entrance conduit which comprises an inwardly tapering section of 1 degree to 4 degrees on each or all sides, most preferably about 1.4-2.6 degrees and more specifically about 1.6 to 1.7 degrees, from its longitudinal axis.
The method of this application comprises passing a tapered cannula through tissue of the patient into the entrance conduit. The tapered cannula may have a proximal end connected with a fluid flow tube, and preferably has a blunt distal end. The cannula may also have an inward taper of 1 to 4 degrees, on each or all sides, from its longitudinal axis between the proximal and distal ends. The inward taper is particularly sized and proportioned to substantially match (that is, be the same angle) and seal with the inwardly tapering section of the entrance conduit. Thus, the cannula can form a readily removable seal with the inwardly tapering section of the entrance conduit, in a manner similar to a well-known luer connector. However, it is believed that an implanted access port to a body lumen has never been used with a tapered cannula to form a luer connection.
Preferably, the tapered cannula has its inward taper over the majority of its length.
In a prior system of Vasca Inc. for connection between implanted ports and tubular sets ending in a cannula, the cannula is initially cylindrical, but makes a seal with an inwardly tapering section of an entrance conduit of an implanted artificial port. Substantial pressure is applied, causing actual deformation of the cannula, on the order of ten pounds applied between the cannula and the tapering section implanted in the patient, to form a seal by cannula deformation.
By this present method, the pressure imposed on the patient to make the connection is on the order of 40 percent or less of such heavy pressures, because by this invention there is no need for deformation of the tapered cannula or the inwardly tapering section to make an effective seal. Furthermore, this makes possible the disconnection and reconnection of the same cannula with the inwardly tapering section of the entrance conduit, since a good seal can be achieved without deformation, and which can be readily broken and then reestablished again if desired.
In order to reduce the torque and other pressures that are needed to break the tapered connection between the cannula of this invention and the inwardly tapering section of the entrance conduit, the inward taper of the tapered cannula may essentially match but be slightly less (on the order of 0.1 degree) than the degree of taper of the inwardly tapering section of the entrance conduit. In this circumstance, a reliable seal can be achieved that is more easily disconnected by twisting and removing of the cannula, when compared with the situation where the tapered cannula and the inwardly tapering section have exactly the same angle of taper. One can reduce this bonding strength of the tapered seal by increasing the difference between the inward taper of the tapered cannula from the degree of taper of the inwardly tapering section. One can also strengthen this bond by reducing the difference between the two tapered angles.
Because of the taper of the tapered cannula over preferably substantially its entire length, the beneficial results previously described of reduced flow resistance can be achieved. Particularly, the tapered cannula may preferably have a distal end of 11-13 G (gauge) and a proximal end of 14-15 G, as measured by the conventional gauge measurement system used in the industry.
The tapered cannula is preferably made of a medically acceptable, rigid material, for example, stainless steel or copolymer plastic.